1. Field of the Invention
The present invention relates to a projection display device which employs a plurality of light emitting diodes for a light source.
2. Description of the Related Art
Conventional projection display devices typically comprise a single high-pressure mercury lamp as a light source which illuminates three or one two-dimensional light modulator, and a projection lens for displaying an image on a screen in an enlarged view.
The high-pressure mercury lamp for use as the light source is an all-round lamp which has a light emission characteristic over a range from ultraviolet to infrared, and is associated with a dichroic mirror, a rotary color filter or the like for selecting wavelength components of emitted white light required for color display to build an illumination optical system for irradiating the two-dimensional light modulator with the light.
The two-dimensional light modulator known in the art may be a transmission-type or a reflection-type liquid crystal panel, a digital mirror device (DMD), and the like. The foregoing projection display device employs a relatively large lamp for the light source because it is generally used in combination with a parabolic or an elliptic reflection mirror. In addition, due to the large amount of heat generated by the lamp, a cooling fan must be used to manage the temperature when the lamp is on. These requirements make it difficult to reduce the size and weight of the overall display device.
JP-2001-249400-A discloses a projection display device for solving the drawbacks of the conventional projection display device.
FIG. 1 illustrates the configuration of the projection display device described in JP-2001-249400-A. As illustrated in FIG. 1, light emitting diode elements or laser diode elements, which are used for light source 901, are arranged in the form of array in correspondence to respective elemental lenses of a pair of fly-eye lenses 902, 903. The resulting optical system is capable of efficiently irradiating two-dimensional light modulator 906 with light flux from the individual light emitting elements. Here, two-dimensional light modulator 906 comprises a transmission-type liquid crystal panel.
Since the projection display device described in JP-2001-249400-A employs light emitting diode elements or laser diode elements for the light source, the overall display device can achieve a reduction in size, weight, and power consumption incurred by the light source. However, when light emitting diode elements or laser diode elements are used for a light source as is the case with the display device exemplified by JP-2001-249400-A, the light source implies an essential problem of ensuring a practical luminance with difficulties.
For example, high-pressure mercury lamps typically used for projection display devices provide a light emission efficiency in a range of 60 to 70 lumens/watt. On the other hand, light emitting diodes, for example, provide a light emission efficiency in a range of 10 to 40 lumens/watt which is significantly lower than that of the high-pressure mercury lamps.
Conventional projection display devices typically employ a light source lamp on the order of 150 to 250 watts of power, wherein the light source lamp of 150 watts, for example, emits light flux exceeding 9,000 lumens. On the other hand, even a recent high-performance light emitting diode of approximately one watt can merely provide the amount of light flux as low as 40 lumens even on the assumption that the light emitting diode provides a light emission efficiency of 40 lumens/watt. For achieving light flux of 9,000 lumens equivalent to a high-pressure mercury lamp, a trial calculation shows that the number of such light emitting diodes required for the purpose amounts to as many as 225.
In a conventional optical system using a fly-eye lens, light emitting elements correspond one-to-one to elemental lenses of the fly-eye lens. As such, the fly-eye lens is required to have the same number of elemental lenses as the number of light emitting elements used in the optical system. This type of light emitting diode has a diameter of approximately 10 mm including an assembled mold armor, so that if 225 light emitting diodes are arranged in matrix, the resulting light source array is unacceptably large, resulting in a failure in reducing the size of the overall projection display device.